The Abc soil types: Podzoluvisols, Albeluvisols or Retisols? A review

ABSTRACT

At an archaeological excavation site in central Belgium, we found whitish soil material interspersing a clay illuviation horizon under a Roman road. Starting from this case, we will illustrate how insights into soil formation and soil geography are relevant for understanding landscape evolution and archaeology. We do this by focusing on the ‘Abc’ soil types, which are silt-loam soils that are well-drained and have a mottled and discontinuous clay illuviation horizon. In Belgium, these soils are, almost exclusively, found under ancient forests. To explain their formation, two hypotheses have been proposed. A first assumes that chemical weathering leads to the degradation of the clay illuviation horizon, a process enhanced by the acidifying effect of forest vegetation. A second hypothesis explains their morphology as relict features from periglacial phenomena. We further review how views on their formation were reflected in Soil Taxonomy (Glossudalfs), the FAO legend of the soil map of the world (Podzoluvisols) and in the World Reference Base for soil resources (Albeluvisols and Retisols). If we accept the hypothesis that the morphology of the Abc soil types has to be attributed to periglacial phenomena, Abc soil types must have been more widespread before deforestation. Agricultural activities promoted the homogenisation of the subsoil and the fading of their morphologic characteristics. A Roman road would have prevented such a homogenisation process. These insights help elucidate the evolution of past and current landscapes.